diff --git a/manual/CMakeLists.txt b/manual/CMakeLists.txt
index 91c30751..53109937 100644
--- a/manual/CMakeLists.txt
+++ b/manual/CMakeLists.txt
@@ -63,15 +63,15 @@ set(DOC_SINGLEHTML_OUTPUT singlehtml/index.html)
 set(DOC_PDF_OUTPUT latex/qpdf.pdf)
 if(BUILD_DOC)
   add_custom_command(OUTPUT ${DOC_HTML_OUTPUT}
-    COMMAND ${SPHINX} -M html ${MANUAL_SRC} ${CMAKE_CURRENT_BINARY_DIR}
+    COMMAND ${SPHINX} -M html ${MANUAL_SRC} ${CMAKE_CURRENT_BINARY_DIR} -W
     COMMAND touch ${DOC_HTML_OUTPUT}
     DEPENDS ${MANUAL_DEPS})
   add_custom_command(OUTPUT ${DOC_SINGLEHTML_OUTPUT}
-    COMMAND ${SPHINX} -M singlehtml ${MANUAL_SRC} ${CMAKE_CURRENT_BINARY_DIR}
+    COMMAND ${SPHINX} -M singlehtml ${MANUAL_SRC} ${CMAKE_CURRENT_BINARY_DIR} -W
     COMMAND touch ${DOC_SINGLEHTML_OUTPUT}
     DEPENDS ${MANUAL_DEPS})
   add_custom_command(OUTPUT ${DOC_PDF_OUTPUT}
-    COMMAND ${SPHINX} -M latexpdf ${MANUAL_SRC} ${CMAKE_CURRENT_BINARY_DIR}
+    COMMAND ${SPHINX} -M latexpdf ${MANUAL_SRC} ${CMAKE_CURRENT_BINARY_DIR} -W
     COMMAND touch ${DOC_PDF_OUTPUT}
     DEPENDS ${MANUAL_DEPS})
   add_custom_target(doc_html DEPENDS ${DOC_HTML_OUTPUT})